This invention relates generally to the production of filamentary material and more particularly to a novel spray spinning nozzle for spinning molten polymers to form a nonwoven structure.
Various apparatus has been developed in the past to create an integrated system for forming a fibrous assembly, such as a nonwoven fabric or the like, directly from a molten filament forming material. Typically, such an apparatus may use an extruder in which one of the various kinds of synthetic resinous polymeric material is melted under the influence of heat and pressure to form a quantity of molten material which can then be forced through a nozzle orifice to form a substantially continuous filament. Typically, each of a plurality of high velocity gaseous jets is directed along the freshly extruded filament at a shallow angle to create a drag force for attenuating the filament. The filament is then carried along by the attenuating gaseous jets and deposited on a collection surface to form a nonwoven structure. Such a device in the past has been known as a spray spinning apparatus because the filamentary material appears to be sprayed against the collection surface.
The atttenuating gaseous jets contribute to filament cooling as well as to attenuating and conveying the filament to the collection surface. Since the filament of polymeric material is still in a somewhat molten or tacky stage as it strikes the collection means or surface, some sticking together occurs at each point where the filament contacts itself. Also, the filament may loop about and stick to itself.
One such spray spinning apparatus is shown in U.S. Pat. No. 3,849,040 which is commonly assigned to the assignee of the present patent application. This patent shows a stream of filamentary material emanating from a nozzle. Two elongated attenuating gas discharge orifices, each with a rectangular cross section, are placed on opposite sides of the nozzle, aligned parallel to one another and positioned forwardly of the nozzle orifice. The jets emanating from the discharge orifices intersect at a point offset from the nozzle axis in the plane of the nozzle axis. The axial component of the drag forces produced on the filament by the gas jets attenuates the filament. The filament is collected on a rotating mandrel against which is biased an idler roller for packing the collected filament into a cylindrical web. One disadvantage of such apparatus relates to the difficulty in controlling the spray pattern. The filament seems to wander, causing an unduly broad and unfocused spray pattern. Thus, great care must be taken to control the geometry of the gas jets to provide a proper distribution in the collected filament. In addition, the intersecting jets diverge from their point of intersection in the direction of the collecting mandrel and produce a relatively wide spray pattern. Since only two vertically aligned planar discharge orifices are used, the resulting attenuating jets tend to control only two degrees of freedom of the filament stream.
Other existing spray spinning nozzles include jets which converge toward a filament in such a manner that the jet axes define a hyperboloid having a waist without intersecting the filament. Such nozzles also induce a twisting motion on the filament which is not a useful and desirable effect. Moreover, as with the planar jets in the nozzle discussed above, the jets diverge downstream of the hyperboloid waist producing a wide spray pattern.
The spray pattern of known nozzles is appreciably larger than the diameter of the collecting mandrel. Accordingly, portions of the filament within the spray pattern but above and below the mandrel will spray past the mandrel and perhaps be collected on the idler roller. This phenomenon is called overspraying. As the idler roller and the mandrel are rotatable, the oversprayed filament may be broken or irregularly compacted. These effects will cause difficulty in controlling the uniformity of the nonwoven structure.
Large spray patterns may also permit the filament to overcool so that it will be somewhat less molten and less tacky when it strikes the collection mandrel and, therefore, less apt to properly bond together so as to form an integrated nonwoven structure. Thus, large spray patterns produce products with poor filament bonding and inferior strength.
A further disadvantage of known spray spinning nozzles is that the gas pressure of the gas jets requires adjustment when the polymeric material flow rate is changed. Thus, careful and time consuming control and adjustment is necessitated.
In most known nozzles, the molten polymeric material and the attenuating gas flow through the same nozzle assembly. Accordingly, the gas jets are not separated from the nozzle orifice by an insulating means to limit heat transfer therebetween. As a result, the gas jets effectively cool the nozzle assembly which may cause polymer freeze-up.
When the jets exhaust upstream of the nozzle orifice, the jets induce a flow of ambient air past the nozzle which may further cool the nozzle convectively and cause the molten polymeric material to harden and obstruct the orifice.
In the past it has been necessary to use high throughput rates of polymeric material through the nozzle to reduce nozzle freeze-up. This produces a thicker stream of freshly extruded filament. Although for some applications, it may be desirable to have a thick filament, it is preferable to be able to produce thin filaments as well. A thicker stream of freshly extruded filament requires higher gas supply pressures to obtain higher momentum from the attenuating gas jet and requires the distance between the nozzle orifice and the collection surface to be greater than desired. As a result of these higher operating parameters, and the difficulty in controlling the spray pattern, the nonwoven structure produced by known spray spinning apparatus has not been entirely satisfactory. The attenuated filament of known spray spinning nozzles includes quantities of "shot" which is a solid debris or bead of nonattenuated polymer which increases cost and weight of the product and undesirably affects the feel of the nonwoven structure. Moreover, the filament thickness for known spray spinning nozzles is a widely varying parameter. This wide variation in the filament thickness also causes a wide variation in filament strength and causes the filament to produce a nonwoven structure with varying strength properties.
Accordingly, it will be apparent to those skilled in the art that the need continues to exist for a spray spinning nozzle which overcomes problems of the types discussed above.